1. Field of the Invention
This invention relates to a combustion method (e.g. for natural gas) and a burner which can be used for the method. In particular the invention is directed to a method in which combustion zones operating in the surface radiant mode are created on the surface of a burner, while at the same time blue flame combustion zones are operated in areas surrounded by the surface radiant zones.
2. Discussion of the Prior Art
Surface combustion radiant burners have been known for some time. Exemplary is U.S. Pat. No. 4,597,734 to McCausland et al., which describes a surface combustion radiant: burner including a porous burner surface made of metal fibers. Porous metal fiber "mats" are advantageous for reasons including their high temperature stability, corrosion resistance, low thermal conductivity, high emissivity and ability to be formed in varying shapes for particular burner applications.
Surface combustion radiant burner designs have been studied to identify designs with greater thermal efficiency and low NO.sub.x emissions. One metal fiber system uses a perforated metal fiber mat, which design has been incorporated in metal fiber burners sold by N. V. Acotech S. A. of Zwevegem, Belgium. Such burners can be run over a broad firing range from the surface radiant mode to the blue flame mode. Studies indicate, however, that no satisfactory solution has been identified for achieving relatively high overall firing rates (e.g. surface firing rates near 1,000,000 btu/hr-ft.sup.2) while at the same time maintaining low NO.sub.x emissions at excess air levels less than 15%. Low excess air operation is necessary to maintain high thermal efficiencies. Further, such studies have identified additional problems such as burner "screechings" when operating at low excess air conditions.